1. Field of Invention
The present invention relates to an apparatus for measuring the current density profile of intense electron beams, but more specifically it relates to an interceptive type electric probe for mapping the radial current density profile of high energy and high current electron beams.
2. Description of the Prior Art
The use of interceptive type probes for measuring the current density profile of electron beams is a common technique in accelerators producing low current, i.e., less than one ampere. Severe requirements are imposed on interceptive devices when current densities of the order of kiloamperes per square centimeter are encountered. In this case, space charge effects, large secondary electron production and probe heating are important considerations.
At the present time, measurement of the beam current density profile of an intense electron beam involves the monitoring of x-rays produced by probes fabricated from high atomic number wires inserted into the electron beam. This measurement system suffers the disadvantages of requiring extensive shielding and elaborate measurement equipment including an x-ray collimator, photomultiplier tube, a fast scintillator and a power supply powering the photomultiplier tube. Accordingly, this type of measurement system is necessarily bulky and expensive.
In practice, when a high energy electron beam hits a high atomic number wire probe, radiation is produced in the x-ray region of the electromagnetic spectrum. The amount of x-radiation produced depends on how much of the electron beam is intercepted by the probe. This measuring technique is very viable for mapping the radial current density profile of an intense electron beam; however, the technique suffers from a number of problems in addition to the previously mentioned ones. One problem is that the materials used to fabricate the probes heat excessively. Thus, from an engineering standpoint, probes made out of these materials have short lives and melt after a short period of time when used in the environment of the intense electron beams. Also, the monitoring system used has to be very sensitive, and, hence, will be sensitive to background x-radiation. This is the reason for the substantial amount of shielding. The shielding is made from lead and has to be configured to surround the monitoring system which makes it bulky, non-portable, very heavy, and, accordingly, very expensive.
Consequently, there is a need in the prior art to configure a probe for measuring the current density of an intense electron beam that is long lived, compact, easy to deploy, portable, and yet inexpensive and requires minimum support equipment for proper operation.
Another technique attempted in the prior art for measuring the beam current of intense electron beams was to make an electrical measurement rather than a measurement of x-rays. As far as it is known, no investigator has obtained an electrical signal, from which useful current density data could be elicited, from an intense electron beam using a bare wire type of interceptive probe. In actual practice, if a bare wire type probe is connected to a coaxial line and interfaced with an oscilloscope, and then inserted into an intense electron beam, a signal is produced which can be observed on the oscilloscope. This signal is very difficult to analyze. One reason is that when the probe is placed into an associated accelerator tube in which the intense electron beam is generated, and the electron beam is not intercepted, a zero response is desirable. On the other hand, when the probe is placed so as to intercept the electron beam an increase in response is desirable. However, with a bare wire the moment that the probe is placed inside the accelerator tube, a response is observed which indicates that there is field coupling between the bare wire probe and the the electron beam within the accelerator tube. The nature of the coupling has been found to be capacitive and, as aforementioned, is field produced.
There is yet another kind of problem that the bare wire electric probe encounters. When an intense electron beam travels through an accelerator tube, it produces a substantial amount of noise of the radio frequency (RF) type. The bare wire electric probe is susceptible to this RF noise which makes the extraction of any useful data from the probe signals about the electron beam very difficult. Thus, the bare wire probe suffers from the undesirable effects of being affected by both electric and magnetic fields which are external to the actual beam charge.
Consequently, there is a need in the prior art to eliminate the effects of field coupling, both electric and magnetic, between a probe for measuring the current density profile of intense electron beams and the electron beam being measured. There is an additional necessity to eliminate the effect of RF type noise which affects the accuracy of the measurements.
There is yet another problem with the bare metal wire probe. When the wire probe is used in the environment of an accelerator tube which pulses on and off very rapidly, there is a tendency for it to melt after continual use. Thus, this probe and the x-ray monitor system which also uses a metal wire probe, are limited in their useful lives.
Consequently, there is a need in the prior art to configure a probe for measuring the beam current density profile of intense electron beams, but yet be configured to have a long life time.
The representative prior art, as outlined hereinabove, include some advances in the measurement of the current density profiles of intense electron beams; however, insofar as can be determined, no prior art device incorporates all of the features and advantages of the present invention.